(a) Field of the Invention
The invention relates to laser cathode-ray tubes (CRT) having an electron gun with a cathode for generating an electron beam, a laser screen, a focusing system for focusing the electron beam on the laser screen and a deflection system for deflecting this beam, and laser cathode ray tubes used, e.g., in projection television systems for displaying images on large square screens or in any field of technology where CRT's are used.
(b) Description of the Related Art
Projection television systems based on conventional cathode-ray tubes having a fluorescent screen are widely used for displaying images on the screens having an area of up to several square meters. However, limited and divergent light beam inherent to such systems makes it difficult to form images of large size with required brightness and contrast. This disadvantage is largely caused by the effect of saturation of the fluorescent intensity resulting from the increase in the current density of the electron beam, as well as by dispersion of the light radiation in the luminophor of a conventional cathode-ray tube.
An effective way to improve the parameters of projection television systems is connected with using laser CRT's (see, for example, U.S. Pat. No. 3,558,956), in which the above-mentioned saturation effect and losses in the luminophor are removed.
As distinct from the conventional CRT's, the source of the radiation in the laser CRT is a laser target that is a thin semiconductor monocrystalline plate having either of its parallel surfaces covered by coatings. A fully reflecting mirror metal coating is usually applied to the surface on which the electron beam is incident, while the other side of the plate is covered with a semitransparent mirror coating. The mirror surfaces constitute an optical resonator, while the semiconductor plate between them acts as an active medium of the laser leading to electron-beam excitation (pumping). The laser target is usually fixed to a transparent substrate serving as the optical output window of the laser CRT and also as a heat sink for the laser target. The substrate can be made of, e.g., sapphire having a high thermal conductivity. The laser target constitutes the screen of the laser CRT (laser screen) together with the transparent substrate.
The electron beam penetrates into the semiconductor plate through the metal coating and induces spontaneous light radiation. When the surface density of the current produced by the beam on the laser target equals a threshold value, the power of the induced light radiation will compensate the losses in the optical resonator and the element of the target on which the electron beam is incident will generate laser radiation. When light passes repeatedly through the resonator, its spectrum narrows, with the result that the emitted light is monochromatic. The laser light is radiated through the semitransparent mirror coating perpendicularly to the surface of the semiconductor plate and leaves the CRT through the sapphire output window.
In U.S. Pat. No. 5,280,360 a laser CRT is described, the CRT including an electron gun with a cathode for generating an electron beam, a laser screen, a focusing system for focusing the electron beam on the laser screen and a deflection system for deflecting this beam.
The method of exciting the screen of the laser CRT according to the above-mentioned patent comprises generating an electron beam and directing this beam to an element of the laser screen for exciting laser radiation. The laser radiation is excited when the current density produced by the electron beam on the element of the screen exceeds a threshold value.
In case of conventional CRT's the increase of the density of electron beam current on each element is useless due to the saturation of fluorescent intensity resulting from the power increase of electron beam. Further, it is known that increase in the current of an electron beam results in its greater diameter because of interaction among the electrons forming the electron beam. Because of this, the increased beam current in the known laser CRT's does not provide a proportional increase in the current density on the laser screen. Therefore, the light radiation intensity does not increase in proportion with the increase of the current. Other negative consequence of the increase in the diameter of the electron beam is reduction in the resolution of the laser CRT.